Historically, packaging grade hot melt adhesive (“HMA”) formulations include a base polymer, and one or more waxes and tackifiers. In addition to basic adhesive bond strength performance, the set-time of the HMA is a key parameter for packaging grade HMAs. This is because lower set times enable faster packaging line speeds.
Set-time is the minimum time interval, after bonding of two substrates, during which the cohesive strength of the bond becomes stronger than the joint stress (often called the pop open force). Set-time thus represents the time necessary to cool down the HMA and obtain a good bond. Open-time is the maximum time interval, after application of the liquid HMA, during which a second substrate can be brought into contact with the molten adhesive and a good bond will be formed. Generally, once the adhesive has cooled to a certain temperature, it is no longer possible to make a bond. Usually HMA formulators attempt to design HMA formulations that have long open time and short setting times, a balance which is difficult to achieve.
Conventional HMA formulations contain up to 30% wax to obtain good set times. Waxes however do not take part in actual adhesive performance; by removing the wax, overall adhesive performance can be enhanced. It would therefore be desirable to provide a hot melt adhesive formulation requiring less wax, preferably little to no wax and higher base polymer loading.